Case hardening



Patented July 21, 1936 CASE HARDENING Edward Van der Pyl, Holden, Masa,assignor to Norton Company, Worcester, Mass, a corporation ofMassachusetts 1% Drawing. Application September 12, 1935,

Serial No. 40,255

6 Claims.

The invention relates to the case hardening of various metals.

One object of the invention is to provide a case hardening methodadapted to produce a particularly hard surface.

Another object of the invention is to provide a case hardening methodwhich shall produce a skin which is highly resistant to corrosion.Another object of the invention is to provide a case hardening method 10susceptible of of soft steels practical application for all types whichshall product a very hard skin thereon at reasonably low cost. Anotherobj ect hardening method of the invention is to provide a case whichshall produce a thick case in a shorter time than heretofore. Anotherobject of the invention is to provide a ferrous article having anextremely tough and hard case. Another object of the invention is tointroduce into a piece of steel not only carbon but also boron. Anotherobject of the invention is to make a steel article which shall be highlyresistant to corrosives.

Other objects will be in part obvious or in part pointed outhereinafter.

The invention accordingly consists in the features'of construction,combination of elements,

arrangements and relation and order of one or more of the othersillustratively described herein, a the application of which of parts,and in the several steps each of said steps to thereof, all as will beand the scope of will be indicated in the following claims.

I provide a carburizing pot Such carburizing pots are known to type.

art and consist formed of any suitable the of highly refractorymaterials into a suitable pot-like shape and having a cover to excludeair. As such a pot might be made of many myself to any particular one,that a refractory 40 1400 C. should be vention in the I further providea which may be other suitable type, and

preferred ferred form of my invention this furnace should be capable ofheating the carburizing pot or pots and the contents thereof to atemperature of 1400 C., maintaining it at that temperature for aconsiderable period of time. ,I need not further describe such furnacesas they are well known.

The method of the invention is particularly applicable to the treatmentof steels as distinguished from cast iron, although I am not,limitedtoany particular ferrous metal, the scope of the invention beingset forth in the following claims.

Assuming, therefore, that it is desired to form a hard case upon a pieceof steel, which may or may not be formed into a particular manufac- 5-tured shape, I select a carbnrizing pot large enough to hold one or anumber of such pieces with ample room for the carburizing material whichshould embed the pieces to be case hardened. Placing in the carburizingpot a 10 quantity of boron carbide (34C) I place the first article to becase hardened in the pot, and fill the pot with articles and boroncarbide so that each one is completely surrounded by the boron carbide.One or more steel articles may be 15 treated at a time. v

I have specified boron carbide of the formula B40 as this is thepreferred way of ca y my invention into effect, but other boron carboncompounds may be used, and also a boron carbide having some othermaterial alloyed therewith, if any such, and for carrying my inventioninto practice I note that boron carbide may be produced according to theUnited States Letters Patent No. 1,897,214.

I may use any grain size of boron carbide or a mixture of various grainsizes. In actual practice I have found No. 16 mesh grain boron carbideto be highly satisfactory, but I do not attribute the advantageousresults of my process to the particular grain size employed and thereappears tometobenoreasonwhyaveryfinemeshgrain would not be equallyadvantageous, and furthermore larger grain sizes can be used. Theselection of grain size will be largely determined by 5 conveniencev ofhandling and cost, although I prefer to avoid very large lumps as it isdesirable to maintain a good contact between the grain and the piecebeing treated. With that end in view a mixture of grain sizes yieldsgood results in so much as a tighter packing may be made of grain inmixed sizes.

I fill the carburizlng pot with articles and boron carbide nearly to thetop thereof, and then cover the boron carbide with a carbonaceoussubstance, such as graphite, lamp black, coke, or the like. I then applythe cover and place the carburizing pot in the furnace. The purpose ofthe carbonaceous substance on the top of the pot is to absorb any oxygenthat may creep by the cover and so prevent it from reaching the boroncarbide, as boron carbide oxidizes under temperatures below thosepreferably employed by me in carrying out this invention.

In the preferred form of the invention I heat the pot and contents toaround 1400" C. For the production of a substantial case upon a smallsteel article, say a rod 6" x 1%" diameter, a treatment under theforegoing conditions for one hour may be employed. After the carburizingpot and contents have been maintained at the desired temperature for thedesired length of time I quench the article or articles in water or oilor in any "other manner according to prior practice.

This may involve a reheating of the objects for convenience, and suchreheating may carry the articles to around 900 C. more or less, as priorpractice may be adhered to so far as the quenching is concerned withexcellent results in my novel process. Under such conditions I haveachieved a hard case upon a 1 diameter rod approximately A in depth.With a longer treatment I can produce a thicker case. It is highlypractical to harden small diameter rods completely according to mymethod, in which event the case is conterminous with the article.Furthermore, in order to produce a tough case upon a piece of steel, Imay bring the pot and contents up to the desired temperature and maycool, in so much as the reaction is extremely fast.

Although I do not wish to be made responsible for theories of whathappens, nevertheless I will now give my present understanding of thenature of the transformation. In so much as iron in all its forms has ahigh aflinity for carbon, which aflinity is greatly increased by hightemperatures, the iron, especially if it be relatively carbon free,draws the carbon atoms from the boron carbide thus disassociating thechemical compound B40 or other boron carbon compound. In drawing thecarbon into itself, however, the iron also draws a large amount ofboron, and possibly the proportionate amount indicated by the formula1340. I believe, however, that the case contains iron and boron andcarbon rather than iron impregnated with boroncarbide, and I believethat an actual dissociation of the boron carbide oc- 'curs. Iron ishardened by the inclusion in solid solution of carbon, which has beenwell known for a long time, but the addition of boron atoms in solidsolution or otherwise greatly increases the hardness of the combination.Boron itself will harden iron but in the practice of my invention it ispreferred to introduce boron by way of boron carbide as described, andiron containing both carbon and boron appears to be harder than ironcontaining boron alone. At all events the result is iron with an alloyof something which is extremely hard and which analytically containsboron and carbon.

I have stated that 1400*0. is the preferred tem-- perature employed incarrying out my invention, audit will beunderstoodthatthisisplus orminusa considerable amount. Awhite heat of the steel is preferably employed.The heat should not be sogreatastomeltthesteehandacoordlngthe meltingpoint of the particularsteel may be considered. It is desirable toattain high as possible without deforming the fleces of turesas'ing Thearticles made according to the invention 5 may be annealed for purposesof destroying the hardness, whereupon they can be bent and worked andthereafter hardened again by, a new heating and quenching. The case orhardened portion is not only hard but also tough and extremely re- 10sistant to corrosive action of acids andv alkalies, and to oxidation andmay be used in any situation where a hard iron or steel article isdesired.

In my copending application Serial No. 36,373, filed August 15, 1935, Ihave described a different 15 method for the production of a steelarticle having a layer of ferrous alloy with boron and carbon, andclaims to the article produced by the method of this present inventionare included in such prior copending application. 20

It will thus be seen that there has been provided by this invention amethod and an article of manufacture in which the various objectshereinabove set forth together with many thoroughly practical advantagesare achieved. As various 25 possible embodiments might be made of thevarious features of the above invention and as the art herein describedmight be varied in various parts, all without departing from the scopeof the invention, it is to be understood that all matter hereinbeforeset forth is to be interpreted as illustrative and not in a limitingsense. In the claims, the word case is to be understood as includingalso the condition in which the case extends to the core of the materialand is conter- 35 minous with it or with a section thereof.

I claim:

1. Method of case hardening steel which comprises placing a' steelarticle in a carburizing pot with comminuted born carbide, heating thesame to a high temperature and quenching the article.

2. Method of case hardening steel which comprises placing a steelarticle in a carburizing pot with boron carbide of the'formula B40,heating to a high temperature and quenching.

3. Method of case hardening steel which comprises placing a steelarticle in a carburizing pot with comminuted boron carbide, heating to1250 C. or above, and quenching.

4. Method'of'case hardening steel which comprises placing a steelarticle in a carburizing pot with comminuted boron carbide, heating toin the neighborhood of 1400 C. and huenching.

5. Method of case hardening ferrous metal which comprises heating themetal in the presence 5 of boron carbide to a temperature at or above1250' C. and quenching. Y

8. Method of case hardening ferrous metal which comprises placing apiece of the metal in a carburizing pot with comminuted boron carbide,

covering the boron carbide with carbonaceous material, heating to a hightemperature and quenchnowsnn' van on m

